Steven Earle

Chapter 3 Summary

The three types of rocks are igneous, formed from magma; sedimentary, formed from fragments of other rocks or precipitations from solution; and metamorphic, formed when existing rocks are altered by heat, pressure, and/or chemical action. The rock cycle summarizes the processes that contribute to cycling of rock material among these three types. The rock cycle is driven by Earth’s internal heat, and by processes happening at the surface, which are driven by solar energy.

3.2

Magma and Magma Formation

Magma is molten rock, and in most cases, it forms from partial melting of existing rock. The two main processes of magma formation are decompression melting and flux melting. Magmas range in composition from ultramafic to felsic. Mafic rocks are rich in iron, magnesium, and calcium and have around 50% silica. Felsic rocks are rich in silica (~75%) and have lower levels of iron, magnesium, and calcium and higher levels of sodium and potassium than mafic rocks.

3.3

Crystallization of Magma

As a body of magma starts to cool, the first process to take place is the polymerization of silica tetrahedra into chains. This increases the magma’s viscosity (makes it thicker) and because felsic magmas have more silica than mafic magmas, they tend to be more viscous. The Bowen reaction series allows us to predict the order of crystallization of magma as it cools. Magma can be modified by fractional crystallization (separation of early-forming crystals) and by incorporation of material from the surrounding rocks by partial melting.

3.4

Classification of Igneous Rocks

Igneous rocks are classified based on their mineral composition and texture. Felsic igneous rocks have less than 20% ferromagnesian silicates (amphibole and/or biotite) plus varying amounts of quartz and both potassium and plagioclase feldspars. Mafic igneous rocks have more than 50% ferromagnesian silicates (primarily pyroxene) plus plagioclase feldspar. Most intrusive igneous rocks are phaneritic (crystals are visible to the naked eye). If there were two stages of cooling (slow then fast), the texture may be porphyritic (large crystals in a matrix of smaller crystals). If water was present during cooling, the texture may be pegmatitic (very large crystals).

3.5

Intrusive Igneous Bodies

Magma intrudes into country rock by pushing it aside or melting through it. Intrusive igneous bodies tend to be either irregular (stocks and batholiths), tabular (dykes and sills), or pipe-like. Batholiths have exposed areas of greater than 100 km2, while stocks are smaller. Sills are parallel to existing layering in the country rock, while dykes cut across layering. A pluton that intruded into cold rock it is likely to have a chilled margin.

Questions for Review

1. What processes must take place to transform rocks into sediment?

2. What processes normally take place in the transformation of sediments to sedimentary rock?

3. What are the processes that lead to the formation of a metamorphic rock?

4. What is the significance of the term reaction in the name of the Bowen reaction series?

5. Why is it common for plagioclase crystals to be zoned from relatively calcium-rich in the middle to more sodium-rich on the outside?

6. What must happen within a magma chamber for fractional crystallization to take place?

7. Explain the difference between aphanitic and phaneritic textures.

8. Explain the difference between porphyritic and pegmatitic textures.

Unless otherwise noted, this book is released under a Creative Commons Attribution 4.0 Unported License also known as a CC-BY license. This means you are free to copy, redistribute, modify or adapt this book.. Under this license, anyone who redistributes or modifies this textbook, in whole or in part, can do so for free providing they properly attribute the book as follows:

Additionally, if you redistribute this textbook, in whole or in part, in either a print or digital format, then you must retain on every physical and/or electronic page the following attribution:

Download this book for free at http://open.bccampus.ca

For questions regarding this licensing, please contact opentext@bccampus.ca. To learn more about BCcampus Open Textbook project, visit http://open.bccampus.ca

Cover image: Mount Robson, British Columbia (3954 m, highest peak in the Canadian Rockies), with the Berg Glacier (left), the Mist Glacier (right) and Berg Lake in the foreground. Mount Robson is almost entirely made up of Cambrian sedimentary rock (ca. 500 Ma) that was pushed eastward and thrust upward during the formation of the Rocky Mountains, mostly during the past 100 million years by Heather Earle is CC BY.